The thing with breast cancer is, I think you will find, that there is a pretty standard process you go thru depending upon your type of cancer. You just need to get thru those crazy busy few days at the start where they run every test known to mankind.
For me, my breast care nurse was with me at each step. So while I was second guessing ten steps ahead, she encouraged me to just work thru the next step.
The start is the hardest. You are in limbo a bit. Once your treatment plan starts then you can start counting the days till it is over. Best wishes.